Ferrite substrate for thin-film inductors, thin-film common mode filter using the substrate, thin-film common mode filter array using the substrate and manufacturing method of the substrate

ABSTRACT

A ferrite substrate for thin-film inductors is provided by means of blending raw materials to meet a composition of di-iron trioxide (Fe 2 O 3 ): 40 to 55 mol %, nickel oxide (NiO): 5 to 35 mol %, zinc oxide (ZnO): 10 to 40 mol %, and bismuth trioxide (Bi 2 O 3 ): 150 to 750 ppm, or of Fe 2 O 3 : 40 to 55 mol %, NiO: 5 to 35 mol %, ZnO: 10 to 40 mol %, cupric oxide (CuO): 5 to 10 mol %, and manganese dioxide (MnO 2 ): 0.5 to 2 mol %, and then molding and sintering the blended material, and applying hot isostatic pressing to the sintered article. A thin-film common mode filter and a thin-film common mode filter array using the ferrite substrate and the manufacturing method of the substrate are also provided.

PRIORITY CLAIM

[0001] This application claims priority from Japanese patent applicationNo. 2003-163563, filed on Jun. 9, 2003, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a ferrite substrate forthin-film inductors, to a thin-film common mode filter using thesubstrate, to a thin-film common mode filter array using the substrate,and to a manufacturing method of the substrate.

[0004] 2. Description of the Related Art

[0005] Common mode filter is a device for suppressing common modecurrents that cause electromagnetic interference in paralleltransmission lines. The common mode filter has magnetically coupledinductors to remove in-phase noise component.

[0006] Thin-film common mode filter miniaturized and highly integratedby forming bilayered thin-film coils between ferrite substrates and byconstructing in chip form, and thin film common mode filter array onwhich a plurality of the filters are mounted, are described in forexample, Japanese Patent Publications Nos. 08-203737A and 11-054326A.

[0007] Generally, such a ferrite substrate is produced by hot formingpress method where a hot-pressed block is cut out into substrates with adesired shape and the substrates are then lapped and formed, or by sheetmanufacturing method where sheeted ferrites are stacked and pressed withheat and the stacked ferrite is then lapped and formed into a desiredshape.

[0008] In the thin-film common mode filter, coils are disposed closelyto each other in order to satisfy it's characteristic request and highvoltage is applied to these coils. Thus, such filter is required to havehigh withstand voltage and high reliability in electrical isolation.Also, required is that terminals of the filter should be electricallyisolated with each other and formed finely without causing electricalisolation failure between coils. Furthermore, the filter should haveminiaturized coils and ferrite substrates with a permeability of about100-400 in order to be operable at a high frequency (several GHz) band.

[0009] Conventional ferrite substrate for the thin-film common modefilter, however, has a porous crystalline structure with voids and suchon its surface, which causes low insulation resistance on its surfaceand large surface-degradation. The ferrite substrate, therefore, has toopoor mechanical strength to undergo thin-film process, and moreover, ithas been difficult to form precisely the terminals on the substratesurface.

BRIEF SUMMARY OF THE INVENTION

[0010] It is therefore an object of the present invention to provide aferrite substrate for thin-film inductors, with a higher surfaceinsulation resistance and less surface-degradation, a thin-film commonmode filter using the substrate, a thin-film common mode filter arrayusing the substrate, and a manufacturing method of the substrate.

[0011] Another object of the present invention is to provide a ferritesubstrate for thin-film inductors with a high mechanical strength, athin-film common mode filter using the substrate, a thin-film commonmode filter array using the substrate, and a manufacturing method of thesubstrate.

[0012] A further object of the present invention is to provide a ferritesubstrate for thin-film inductors, on a surface of which terminals canbe precisely formed without difficulty, a thin-film common mode filterusing the substrate, a thin-film common mode filter array using thesubstrate, and a manufacturing method of the substrate.

[0013] According to the present invention, a ferrite substrate forthin-film inductors is provided, which contains a ferrite composition ofdi-iron trioxide (Fe₂O₃): 40 to 55 mol %, nickel oxide (NiO): 5 to 35mol %, zinc oxide (ZnO): 10 to 40 mol %, and bismuth trioxide (Bi₂O₃):150 to 750 ppm, or of Fe₂O₃: 40 to 55 mol %, NiO: 5 to 35 mol %, ZnO: 10to 40 mol %, cupric oxide (CuO): 5 to 10 mol %, and manganese dioxide(MnO₂): 0.5 to 2 mol %, and which has a densified crystalline structuredeveloped by hot isostatic pressing (HIP). Also, a thin-film common modefilter and a thin-film common mode filter array, which are produced froma part of the substrate, are provided.

[0014] More preferably, the substrate contains a ferrite composition ofFe₂O₃: 40 to 55 mol %, NiO: 15 to 30 mol %, ZnO: 20 to 40 mol %, andBi₂O₃: 150 to 750 ppm, or of Fe₂O₃: 40 to 55 mol %, NiO: 15 to 30 mol %,ZnO: 20 to 40 mol %, CuO: 5 to 10 mol %, and MnO₂: 0.5 to 2 mol %.

[0015] By being given the crystalline structure densified by HIP withthe above-mentioned ferrite composition, the substrate achieves highsurface insulation resistance of 2×10¹⁰ Ω·cm or more, and the commonmode filter produced from the substrate can acquire enough electricalisolation between the coils. And, there is no change (degradation) inbulk insulation resistance and surface insulation resistance of thesubstrate after being annealed in the thin film process. Further,mechanical strength (bending strength) of the substrate is enhanced tothe value at least 1.5 times larger than that of substrate produced byconventional hot forming press method, which is a enough strength forthe substrate to undergo the thin film process. Furthermore, thedensified substrate-surface with almost no voids can prevent electricaltrouble due to plating on unwanted portion when the terminals and thelike are formed by plating. In addition, the terminal patterns are ableto being formed precisely because of the densified substrate-surface.

[0016] Preferably, the substrate is a wafer with diameter of 3 inches ormore.

[0017] According to the present invention, a manufacturing method of aferrite substrate for thin film inductors is further provided, whichincludes a step of blending, and adding if needed, raw materials to meeta composition of Fe₂O₃: 40 to 55 mol %, NiO: 5 to 35 mol %, ZnO: 10 to40 mol %, and Bi₂O₃: 150 to 750 ppm, or of Fe₂O₃: 40 to 55 mol %, NiO: 5to 35 mol %, ZnO: 10 to 40 mol %, CuO: 5 to 10 mol %, and MnO₂: 0.5 to 2mol %, and a step of molding and sintering the blended material, andthen applying HIP to the sintered article.

[0018] More preferably, the method includes a step of blending rawmaterials to meet a composition of Fe₂O₃: 40 to 55 mol %, NiO: 15 to 30mol %, ZnO: 20 to 40 mol %, and Bi₂O₃: 150 to 750 ppm, or of Fe₂O₃: 40to 55 mol %, NiO: 15 to 30 mol %, ZnO: 20 to 40 mol %, CuO: 5 to 10 mol%, and MnO₂: 0.5 to 2 mol %.

[0019] By undergoing HIP, after being set into the above-mentionedferrite composition and sintered, the obtained substrate achieves a highsurface insulation resistance value of 2×10¹⁰ Ω·cm or more, and thecommon mode filter produced from the substrate can acquire enoughelectrical isolation between the coils. Further, there is no change ordegradation in bulk insulation resistance and surface insulationresistance of the substrate after being annealed in the thin filmprocess. Further, mechanical strength or bending strength of thesubstrate is enhanced to the value at least 1.5 times larger than thatof substrate produced by conventional hot forming press method, which isa enough strength for the substrate to undergo the thin film process.Furthermore, the densified substrate-surface with almost no voids canprevent electrical trouble due to plating on unwanted portion when theterminals and like are formed by plating. In addition, the terminalpatterns are able to being formed precisely because of the densifiedsubstrate-surface.

[0020] Preferably, the method further includes a step of annealing theobtained article and surface-lapping the annealed article with theamount of lapping at least 5 μm after applying HIP to the article.

[0021] Further objects and advantages of the present invention will beapparent from the following description of the preferred embodiments ofthe invention as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0022]FIG. 1 shows a flow diagram schematically illustrating some stepsin a preferred embodiment of a manufacturing method of a ferritesubstrate for thin-film inductors according to the present invention;

[0023]FIGS. 2a to 2 j show perspective views for explanation of a waferprocess to produce a thin-film common mode filter array;

[0024]FIGS. 3a to 3 j show perspective views for explanation of aworking process to produce the thin-film common mode filter array;

[0025]FIG. 4 shows a graph illustrating common mode characteristic of athin-film common mode filter;

[0026]FIG. 5 shows a graph illustrating a relationship among Fe₂O₃, NiOand ZnO compositions and permeability μ in the ferrite substrates;

[0027]FIG. 6 shows a graph illustrating the measurement results ofsurface insulation resistance of the ferrite substrates containingvarious Fe₂O₃ contents after being sintered;

[0028]FIG. 7 shows a graph illustrating the measurement results ofsurface insulation resistance of the sintered ferrite substrates shownin FIG. 6 after being surface-lapped;

[0029]FIG. 8 shows a graph illustrating the measurement results ofsurface insulation resistance of the ferrite substrates shown in FIG. 7after being annealed 5 times at a curing temperature of insulatinglayers (about 400° C.);

[0030]FIG. 9 shows a graph illustrating the measurement results ofsurface insulation resistance of the ferrite substrates shown in FIG. 8after being annealed in vacuum at 1000° C.;

[0031]FIG. 10 shows a graph illustrating the measurement results ofsurface insulation resistance of the annealed-in-vacuum ferritesubstrates shown in FIG. 9 after being surface-lapped with the amount oflapping of 5 μm or more;

[0032]FIG. 11 shows a graph illustrating the measurement results of therelationship between the amount of lapping and surface resistance of thesubstrate shown in FIG. 10;

[0033]FIG. 12 shows a graph illustrating the measurement results ofsurface insulation resistance of the surface-lapped ferrite substratesshown in FIG. 10 after being annealed 5 times in vacuum at about 400°C.;

[0034]FIG. 13 shows a graph illustrating the measurement results ofsurface insulation resistance of the ferrite substrates shown in FIG. 12and NiZn-ferrite substrates produced with addition of Bi₂O₃ to theirbasic composition;

[0035]FIG. 14 shows a graph illustrating the measurement results of therelationship between the amount of added Bi₂O₃ and insulation resistancein the ferrite substrate with Fe₂O₃ 55 mol %;

[0036]FIG. 15 shows a graph illustrating the measurement results of therelationship between the amount of added Bi₂O₃ and bending strength inthe ferrite substrate with Fe₂O₃ 55 mol %;

[0037]FIG. 16 shows a graph illustrating the measurement results of therelationship between bending strength and the amount of added CuO in theNiZn-ferrite substrates produced with addition of CuO to their basiccomposition;

[0038]FIG. 17 shows a graph illustrating the measurement results of therelationship between permeability μ and the amount of added MnO₂ in thesubstrates produced with addition of MnO₂ to the composition shown inFIG. 16;

[0039]FIG. 18 shows a graph illustrating the measurement results of therelationship between insulation resistance and the amount of added MnO₂in the substrates produced with addition of MnO₂ to the compositionshown in FIG. 16;

[0040]FIG. 19 shows a graph illustrating the measurement results of therelationship between insulation resistance and the amount of added Bi₂O₃in the substrates produced by HP and those produced by HIP;

[0041]FIG. 20 shows a graph illustrating the measurement results therelationship between bending strength and the amount of added Bi₂O₃ inthe substrates produced by HP and those produced by HIP;

[0042]FIG. 21 shows a graph illustrating the measurement results of therelationship between applied pressure in HIP and bending strength of thesubstrates;

[0043]FIG. 22 shows a graph illustrating the measurement results therelationship between applied pressure in HP and bending strength of thesubstrates;

[0044]FIG. 23 shows a graph illustrating the measurement results therelationship between density and applied pressure in the ferritesubstrates produced by HP and the ferrite substrates produced by HIP,both of which consist basically of Fe₂O₃, NiO and ZnO;

[0045]FIG. 24 shows a graph illustrating the measurement results of therelationship between surface roughness and grit number of the usedabrasive grain, in the ferrite substrates produced by HP and the ferritesubstrates produced by HIP, both of which consist basically of Fe₂O₃,NiO and ZnO;

[0046]FIG. 25 shows an optical microscope photograph of the surface ofthe ferrite substrate that was produced by HIP;

[0047]FIG. 26 shows an optical microscope photograph of the surface ofthe ferrite substrate that was produced by HP; and

[0048]FIG. 27 shows an optical microscope photograph of the surface ofthe ferrite substrate that was produced by sheet manufacturing method.

DETAILED DESCRIPTION OF THE INVENTION

[0049]FIG. 1 schematically illustrates some steps in a preferredembodiment of a manufacturing method of a ferrite substrate forthin-film inductors according to the present invention. Themanufacturing steps of the ferrite substrate will be described in detailwith reference to the figure hereafter.

FIRST EXAMPLE OF THE SUBSTRATE COMPOSITION

[0050] First, raw materials are weighed according to the blend table sothat obtained ferrite substrates will have a predetermined composition,and then are blended by adding purified water (step S1). Thepredetermined composition is Fe₂O₃: 40 to 55 mol %, NiO: 15 to 30 mol %,and ZnO: 20 to 40 mol %.

[0051] Next, the obtained blended slurry is dried (step S2), andpresintered (step S3).

[0052] Then, the obtained presintered material is milled with purifiedwater (step S4). The milling is accompanied by adding 150 to 750 ppm inBi₂O₃. CaCO₃ and such also may be added.

[0053] Next, the obtained milled material is dried and granulated, andthen is molded (step S5). Further, it is sintered (step S6), in theatmospheric air as combustion gas at about 1160° C.

[0054] Then, the sintered article undergoes HIP (step S7) that isperformed for about 2 hours under the pressure of about 1000 kg/cm² atabout 1200° C.

[0055] Then, the obtained article is plane-grinded, shaped and cut (stepS8).

[0056] Thereafter, the cut article is heated or annealed (step S9), inthe atmospheric air at about 1000° C.

[0057] Then, the surface of the annealed article is lapped with theamount of lapping of at least 5 μm, by use of abrasive grain of gritnumber #2000 (step S10).

SECOND EXAMPLE OF THE SUBSTRATE COMPOSITION

[0058] First, raw materials are weighed according to the blend table sothat obtained ferrite substrates will have a predetermined composition,and then are blended by adding purified water (step S1). Thepredetermined composition is Fe₂O₃: 40 to 55 mol %, NiO: 15 to 30 mol %,ZnO: 20 to 40 mol %, CuO: 5 to 10 mol %, and MnO₂: 0.5 to 2 mol %.

[0059] Next, the obtained blended slurry is dried (step S2), andpresintered (step S3).

[0060] Then, the obtained presintered material is milled with purifiedwater (step S4). The milling may be accompanied by adding CaCO₃ andsuch.

[0061] Next, the obtained milled material is dried and granulated, andthen is molded (step S5). Further, it is sintered (step S6), in theatmospheric air as combustion gas at about 1160° C.

[0062] Then, the sintered article undergoes HIP (step S7) that isperformed for about 2 hours under the pressure of about 1000 kg/cm² atabout 1200° C.

[0063] Then, the obtained article is plane-grinded, shaped and cut (stepS8).

[0064] Thereafter, the cut article is heated or annealed (step S9), inthe atmospheric at 1000° C.

[0065] Then, the surface of the annealed article is lapped with theamount of lapping of at least 5 μm, by use of abrasive grain #2000 (stepS10).

[0066] By undergoing HIP, after being set into the above-mentionedferrite composition and sintered, and by being annealed andsurface-lapped as mentioned above, the obtained substrate achieves ahigh surface insulation resistance value of 2×10¹⁰ Ω·cm or more.Further, there is no change (degradation) in bulk insulation resistanceand surface insulation resistance in the substrate after being annealedin the thin film process thereafter. Further, mechanical strength(bending strength) of the substrate is enhanced to the value at least1.5 times larger than that of substrate produced by conventional hotforming press method, which is enough strength for the substrate toundergo the thin film process. Furthermore, the substrate surfacebecomes densified with almost no voids, as well as the surface in theproduction process.

[0067]FIGS. 2a to 2 j and FIGS. 3a to 3 j show perspective views forexplanation of the wafer process and the working process to produce athin film common mode filter array that consists of two coupled thinfilm common mode filters, fabricated from the above-mentioned ferritesubstrate. In FIGS. 2a-2 j and FIGS. 3a-3 j, the lower parts of the viewshow a wafer, and the upper parts show individual chips that are notactually cut to separate. The manufacturing process of the thin filmcommon mode filter array will be detailed by these figures hereafter.

[0068] First, as shown in FIG. 2a, a ferrite wafer that was fabricatedby the manufacturing method of FIG. 1 is prepared, and, as shown in FIG.2b, a first insulating layer 21, made of such as polyimide resin, iscoated on the wafer 20, and is then patterned.

[0069] Next, as shown in FIG. 2c, first leads and electrodes of a copperlayer 22 are formed on the first insulating layer 21. Then, as shown inFIG. 2d, a second insulating layer 23, made of such as polyimide resin,is coated thereon, and patterned.

[0070] Then, as shown in FIG. 2e, first coils of a copper layer 24 areformed on the second insulating layer 23. Then, as shown in FIG. 2f, athird insulating layer 25, made of such as polyimide resin, is coatedthereon, and patterned.

[0071] Then, as shown in FIG. 2g, second coils of a copper layer 26 areformed on the third insulating layer 25. Then, as shown in FIG. 2h, afourth insulating layer 27, made of such as polyimide resin, is coatedthereon, and patterned.

[0072] Then, as shown in FIG. 2i, second leads of a copper layer 28 areformed on the fourth insulating layer 27. Then, as shown in FIGS. 2j and3 a, a fifth insulating layer 29, made of such as polyimide resin, iscoated thereon, and patterned.

[0073] After that, as shown in FIG. 3b, a silver paste 30 isscreen-printed on the leads. Then, as shown in FIG. 3c, a ferrite paste31 for flux return portion is embedded in the core portions.

[0074] Then, as shown in FIG. 3d, a ferrite plate cover 32 is bonded onthe processed wafer with adhesive.

[0075] Then, as shown in FIG. 3e, the obtained wafer is cut into bars 33on each of which a plurality of thin film common mode filter array chipsare aligned.

[0076] Then, as shown in FIG. 3f, a mark 34 is printed on the upper sideof each of the thin film common mode filter array chips in the bar 33.Then, as shown in FIG. 3g, electrode terminals 35 of Nickel are formedby sputtering on the side of each of the thin film common mode filterarray chips in the bar 33.

[0077] After that, as shown in FIG. 3h, each bar is cut to separate intoindividual chips 36. Then, as shown in FIG. 3i, the electrode terminals35 are formed into bilayer structure 37 of a Nickel layer and a tinlayer by barrel plating. Further, as shown in FIG. 3j, the obtained thinfilm common mode filter array chips 36 are bonded on a tape 38.

[0078] The ferrite substrate is required to have high electricalinsulation performance in bulk because, as shown in FIG. 3g, the thinfilm common mode filter and thin film common mode filter array have theelectrode terminals formed on the cut surface of the ferrite substrate.And the ferrite substrate is also required to have high surfaceinsulation performance. The thin film common mode filter produced fromthe substrates is required to have insulation resistance on the order of10⁸ Ω between the coil terminals. Although there is no perfectproportionality relation between the substrate surface resistance andactual terminal-to-terminal resistance, the substrate is required tohave 2×10¹⁰ Ω or more of the combined resistance of the bulk resistanceand the surface resistance, to guarantee at least 10⁸ Ω of theinsulation resistance. Furthermore, the ferrite substrate is alsorequired to maintain stably high surface insulation performance duringsuch a heat process that is performed in the atmospheric air or Nitrogengas at more or less 400° C. for heat cure of the insulating layer in thewafer process for forming the thin film common mode filter.

[0079] In addition, the ferrite substrate is required not to be crackedor so by mechanical shock or thermal shock in the wafer process because,as shown in FIGS. 2a-2 j, the thin film common mode filter and thin filmcommon mode filter array are formed all together on the ferritesubstrate. The resistance to such a crack depends on the bendingstrength of the substrate, therefore the ferrite substrate is requiredto have higher bending strength. Particularly, the larger is the size ofthe substrate, the higher bending strength the substrate must have, toenhance its resistance to crack.

[0080] Further, as mentioned above, thin film micropatterns are formedon the ferrite substrate in the production process of the thin filmcommon mode filter and thin film common mode filter array, therefore thecoated film on the substrate is required not to billow, and themicropatterns are required not to be deformed, due to rough surface ofthe substrate or so. Usually, smaller degree of surface roughness of thesubstrate than the thickness of the coated film is required for carryingout the thin film process. For example, in the process of coating thepolyimide films, patterning is difficult to be performed on thesubstrate with surface roughness Rmax of 6 μm or more.

[0081]FIG. 4 shows a graph illustrating common mode characteristic of athin film common mode filter fabricated by the above-mentioned process,that is, frequency dependence of intrinsic impedance Z.

[0082] As understood from FIG. 4, the common mode filters, using thesubstrates made of ferrite materials with various permeability μ about100 to 1400, acquire almost the same common mode characteristic.

[0083]FIG. 5 shows a graph illustrating the relationship among Fe₂O₃,NiO and ZnO compositions and permeability μ in the ferrite substrates.

[0084] As clarified from in FIG. 5, to meet the common mode impedancecharacteristic of the thin film common mode filter shown in FIG. 4, theferrite substrate is required to contain a composition in the range ofFe₂O₃: 40 to 70 mol %, NiO: 5 to 35 mol %, ZnO: 10 to 40 mol %.

[0085]FIG. 6 shows a graph illustrating the measurement results ofsurface insulation resistance of the ferrite substrates just after beingsintered, which contain various Fe₂O₃ contents. And FIG. 7 shows a graphillustrating the measurement results of surface insulation resistance ofthe sintered ferrite substrates after being surface-lapped.

[0086] As understood from FIG. 6, the NiZn ferrite substrates just afterbeing sintered indicate greatly high surface insulation resistance of10¹² Ω or more in the Fe₂O₃ content range from 30 to 65 mol %. Further,the sintered substrates after being surface-lapped maintain greatly highsurface insulation resistance.

[0087]FIG. 8 shows a graph illustrating the measurement results ofsurface insulation resistance of the ferrite substrates shown in FIG. 7,which contain various Fe₂O₃ contents, after being annealed (5 times) ata curing temperature of insulating layers (about 400° C.). And FIG. 9shows a graph illustrating the measurement results of surface insulationresistance of these ferrite substrates after being annealed in vacuum at1000° C.

[0088] As understood from FIG. 8, the substrates annealed repeatedly atthe curing temperature of insulating layer show degraded surfaceinsulation performance down to the order of 10⁹ Ω in surface insulationresistance. Further, as understood from FIG. 9, the substrates annealedin vacuum show greatly degraded surface insulation performance down tothe order of 10⁸ Ω in surface insulation resistance.

[0089]FIG. 10 shows a graph illustrating the measurement results ofsurface insulation resistance of the annealed-in-vacuum ferritesubstrates shown in FIG. 9 after being surface-lapped with the amount of5 μm or more.

[0090] From FIG. 10, it is noticed that the substrates reacquire greatlyhigh surface insulation resistance by being surface-lapped. From thisfact, understood is that the decrease in resistance is associated withthe surface condition of the ferrite substrate.

[0091]FIG. 11 shows a graph illustrating the measurement results of therelationship between the amount of lapping and surface resistance.

[0092] From FIG. 11, it is noticed that the surface resistance risessharply over 5 μm of the amount of lapping. Therefore, preferable isthat the amount of surface lapping is set at 5 μm or more.

[0093]FIG. 12 shows a graph illustrating the measurement results ofsurface insulation resistance of the surface-lapped ferrite substratesshown in FIG. 10 after being annealed 5 times in vacuum at a curingtemperature of insulating layers (about 400° C.).

[0094] As evidenced by comparing FIG. 12 with FIG. 8, by beingsurface-lapped with the amount of 5 μm or more, the ferrite substratesannealed for curing insulating layers show smaller decrease in surfaceresistance. Especially, the substrates indicate high surface resistancevalues of 10¹⁰ Ω or more in the Fe₂O₃ content range of 55 mol % or less.The insulation resistance is dropped sharply over the Fe₂O₃ contentrange.

[0095] Therefore, to guarantee the resistance of at least 2×10¹⁰ Ω, thesubstrate should have a Fe₂O₃ content of 55 mol % or less. Further,according to the measurement results of permeability μ shown in FIG. 5,more preferable is that the substrates have a composition of Fe₂O₃: 40to 55 mol %, NiO: 15 to 30 mol %, ZnO: 20 to 40 mol %.

[0096]FIG. 13 shows a graph illustrating the measurement results ofsurface insulation resistance of the ferrite substrates shown in FIG. 12and NiZn-ferrite substrates produced with addition of Bi₂O₃ to theirbasic composition of Fe₂O₃, NiO and ZnO. Line a corresponds to theBi₂O₃-added substrates, and line b corresponds to the no-Bi₂O₃-addedsubstrates, both of which were annealed in vacuum at 1000° C. and thenwere surface-lapped with the amount of 8 μm.

[0097] It is noted that the surface insulation resistance increases byadding Bi₂O₃.

[0098]FIG. 14 shows a graph illustrating the measurement results of therelationship between the amount of added Bi₂O₃ and insulation resistancein the ferrite substrate with Fe₂O₃ content of 55 mol %.

[0099] From FIG. 14, it is noticed that the insulation resistance isgreatly improved by adding 150 ppm or more of Bi₂O₃.

[0100]FIG. 15 shows a graph illustrating the measurement results of therelationship between the amount of added Bi₂O₃ and bending strength inthe ferrite substrate with Fe₂O₃ content of 55 mol %. The measurementwas based on JIS transverse test. The span of the measuring object was1.4 mm, and the weighing rate was 30 mm/min.

[0101] From FIG. 15, it is noticed that the bending strength fallssharply by adding 750 ppm or more of Bi₂O₃.

[0102] As mentioned above, understood is that the insulation resistanceand the bending strength are optimized together by adding 150 to 750 ppmof Bi₂O₃ as the first example of the substrate composition.

[0103]FIG. 16 shows a graph illustrating the measurement results of therelationship between bending strength and the amount of added CuO in theNiZn-ferrite substrates produced with addition of CuO to their basiccomposition of Fe₂O₃, NiO and ZnO.

[0104] From FIG. 16, it is noticed that the bending strength increaseswith the amount of added CuO from 5 to 10 mol %.

[0105]FIG. 17 shows a graph illustrating the measurement results of therelationship between permeability μ and the amount of added MnO₂ in thesubstrates produced with addition of MnO₂ to the composition shown inFIG. 16. And FIG. 18 shows a graph illustrating the measurement resultsof the relationship between insulation resistance and the amount ofadded MnO₂ in the substrates produced with addition of MnO₂ to thecomposition shown in FIG. 16.

[0106] As clarified from FIG. 17, the permeability μ increases with theamount of added MnO₂ from 0.5 to 5 mol %. However, the substrateinsulation resistance falls sharply by adding 2 mol % or more ofMnO_(2,) as shown in FIG. 18.

[0107] Therefore, the bending strength and the permeability μ can beimproved together without the insulation resistance decrease, by adding5 to 10 mol % of CuO and 0.5 to 2 mol % of MnO₂ as the second example ofthe substrate composition.

[0108]FIG. 19 shows a graph illustrating the measurement results of therelationship between insulation resistance and the amount of addedBi₂O₃, in the ferrite substrates produced by conventional hot formingpress method (HP) and the ferrite substrates produced by HIP accordingto the invention. And FIG. 20 shows a graph illustrating the measurementresults of the relationship between bending strength and the amount ofadded Bi₂O₃, in the ferrite substrates produced by HP and the ferritesubstrates produced by HIP.

[0109] As shown in FIG. 19, there is little difference in the insulationresistance between the ferrite substrates produced by HP and thoseproduced by HIP. However, as shown in FIG. 20, the bending strength ofthe substrate produced by HIP is about time and a half larger than thatof the substrate produced by HP. That is, the substrate produced by HIPis harder to crack. The tendency becomes marked as the wafer sizebecomes larger.

[0110]FIG. 21 shows a graph illustrating the measurement results of therelationship between applied pressure in HIP and bending strength of thesubstrates. And FIG. 22 shows a graph illustrating the measurementresults of the relationship between applied pressure in HP and bendingstrength of the substrates. The processing temperature in both HP andHIP was 1200° C.

[0111] As shown in FIG. 21, it is noticed that the substrates acquirelarge bending strengths by undergoing HIP under the HIP pressure of 0.5t/cm² or more. On the other hand, the substrate cannot acquire so largebending strengths by undergoing HP under the increased HP pressure, asshown in FIG. 22.

[0112] Table 1 illustrates the observation results of crack occurrencefrequency in the 3-inch and 6-inch ferrite substrates (with thickness of2 mm) produced by HP and the 3-inch and 6-inch ferrite substratesproduced by HIP, both of which repeatedly underwent 10 times thermalshocks at 110° C. and 10 times sets of suction and detaching in thecarrying process. The number of sample was 20.

[0113] The 3-inch substrates produced by HIP show less crack occurrencefrequency than those produced by HP. In the 6-inch substrates, there isa larger difference of the frequency between the substrates by HP andthose by HIP. TABLE 1 1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 3-inch HP0 0 1 1 1 2 2 2 2 2 wafer HIP 0 0 0 0 0 0 0 0 0 0 6-inch HP 1 4 6 6 8 912 15 18 19 wafer HIP 0 0 0 0 0 0 0 0 0 1

[0114]FIG. 23 shows a graph illustrating the measurement results of therelationship between density and applied pressure in the ferritesubstrates produced by HP and the ferrite substrates produced by HIP,both of which consist basically of Fe₂O₃, NiO and ZnO.

[0115] As shown in FIG. 23, it is noticed that the substrate density isimproved by applying HIP to the substrate.

[0116]FIG. 24 shows a graph illustrating the measurement results of therelationship between surface roughness and grit number of the usedabrasive grain, in the ferrite substrates produced by HP and the ferritesubstrates produced by HIP, both of which consist basically of Fe₂O₃,NiO and ZnO. The crystalline grain size of both ferrite substrates was 5μm, and #2000 SiC was used as abrasive grain for lapping of bothsubstrates.

[0117] As shown in FIG. 24, it is noticed that the surface roughnessdecreases greatly by application of HIP to the substrate. Further, thesurface roughness of the substrates produced by HP shows less change asthe used abrasive grain becomes fine according to the grit number changefrom #1200 to #2000 and to #4000, whereas the surface roughness of thesubstrates produced by HIP is improved as the used abrasive grainbecomes fine.

[0118]FIG. 25 shows an optical microscope photograph (×220) of thesurface of the substrate that was processed by HIP and was lapped with#6000 diamond, and FIG. 26 shows an optical microscope photograph (×220)of the surface of the substrate that was processed by HP and lapped with#6000 diamond. Further, FIG. 27 shows an optical microscope photograph(×220) of the surface of the substrate that was processed by sheetmanufacturing method and was lapped with #6000 diamond. The crystallinegrain sizes of all the ferrite substrates were 5-6 μm.

[0119] The surface of the ferrite substrate produced by HIP as shown inFIG. 25 has almost no pin holes, whereas the surface of the ferritesubstrate produced by conventional HP as shown in FIG. 26 or produced byHP after sheet manufacturing has some voids. Further, the surface of theferrite substrate produced by sheet manufacturing method as shown inFIG. 27 has some large vacancies from which the ferrite particles weredetached.

[0120] All the foregoing embodiments are by way of example of thepresent invention only and not intended to be limiting, and many widelydifferent alternations and modifications of the present invention may beconstructed without departing from the spirit and scope of the presentinvention. Accordingly, the present invention is limited only as definedin the following claims and equivalents thereto.

1. A ferrite substrate for thin film inductors containing a ferritecomposition of di-iron trioxide: 40 to 55 mol %, nickel oxide: 5 to 35mol %, zinc oxide: 10 to 40 mol %, and bismuth trioxide: 150 to 750 ppm,and having densified crystalline structure developed by hot isostaticpressing.
 2. The ferrite substrate as claimed in claim 1, wherein saidferrite composition is di-iron trioxide: 40 to 55 mol %, nickel oxide:15 to 30 mol %, zinc oxide: 20 to 40 mol %, and bismuth trioxide: 150 to750 ppm.
 3. The ferrite substrate as claimed in claim 1, wherein saidsubstrate is a wafer with diameter of 3 inches or more.
 4. A ferritesubstrate for thin film inductors containing a ferrite composition ofdi-iron trioxide: 40 to 55 mol %, nickel oxide: 5 to 35 mol %, zincoxide: 10 to 40 mol %, cupric oxide: 5 to 10 mol %, and manganesedioxide: 0.5 to 2 mol %, and having densified crystalline structuredeveloped by hot isostatic pressing.
 5. The ferrite substrate as claimedin claim 4, wherein said ferrite composition is di-iron trioxide: 40 to55 mol %, nickel oxide: 15 to 30 mol %, zinc oxide: 20 to 40 mol %,cupric oxide: 5 to 10 mol %, and manganese dioxide: 0.5 to 2 mol %. 6.The ferrite substrate as claimed in claim 4, wherein said substrate is awafer with diameter of 3 inches or more.
 7. A thin film common modefilter using a ferrite substrate containing a ferrite composition ofdi-iron trioxide: 40 to 55 mol %, nickel oxide: 5 to 35 mol %, zincoxide: 10 to 40 mol %, and bismuth trioxide: 150 to 750 ppm, and saidsubstrate having densified crystalline structure developed by hotisostatic pressing.
 8. The thin film common mode filter as claimed inclaim 7, wherein said ferrite composition is di-iron trioxide: 40 to 55mol %, nickel oxide: 15 to 30 mol %, zinc oxide: 20 to 40 mol %, andbismuth trioxide: 150 to 750 ppm.
 9. The thin film common mode filter asclaimed in claim 7, wherein said substrate is a wafer with diameter of 3inches or more.
 10. A thin film common mode filter using a ferritesubstrate containing a ferrite composition of di-iron trioxide: 40 to 55mol %, nickel oxide: 5 to 35 mol %, zinc oxide: 10 to 40 mol %, cupricoxide: 5 to 10 mol %, and manganese dioxide: 0.5 to 2 mol %, and saidsubstrate having densified crystalline structure developed by hotisostatic pressing.
 11. The thin film common mode filter as claimed inclaim 10, wherein said ferrite composition is di-iron trioxide: 40 to 55mol %, nickel oxide: 15 to 30 mol %, zinc oxide: 20 to 40 mol %, cupricoxide: 5 to 10 mol %, and manganese dioxide: 0.5 to 2 mol %.
 12. Thethin film common mode filter as claimed in claim 10, wherein saidsubstrate is a wafer with diameter of 3 inches or more.
 13. A thin filmcommon mode filter array using a ferrite substrate containing a ferritecomposition of di-iron trioxide: 40 to 55 mol %, nickel oxide: 5 to 35mol %, zinc oxide: 10 to 40 mol %, and bismuth trioxide: 150 to 750 ppm,and said substrate having densified crystalline structure developed byhot isostatic pressing.
 14. The thin film common mode filter array asclaimed in claim 13, wherein said ferrite composition is di-irontrioxide: 40 to 55 mol %, nickel oxide: 15 to 30 mol %, zinc oxide: 20to 40 mol %, and bismuth trioxide: 150 to 750 ppm.
 15. The thin filmcommon mode filter array as claimed in claim 13, wherein said substrateis a wafer with diameter of 3 inches or more.
 16. A thin film commonmode filter array using a ferrite substrate containing a ferritecomposition of di-iron trioxide: 40 to 55 mol %, nickel oxide: 5 to 35mol %, zinc oxide: 10 to 40 mol %, cupric oxide: 5 to 10 mol %, andmanganese dioxide: 0.5 to 2 mol %, and said substrate having densifiedcrystalline structure developed by hot isostatic pressing.
 17. The thinfilm common mode filter array as claimed in claim 16, wherein saidferrite composition is di-iron trioxide: 40 to 55 mol %, nickel oxide:15 to 30 mol %, zinc oxide: 20 to 40 mol %, cupric oxide: 5 to 10 mol %,and manganese dioxide: 0.5 to 2 mol %.
 18. The thin film common modefilter array as claimed in claim 16, wherein said substrate is a waferwith diameter of 3 inches or more.
 19. A manufacturing method of aferrite substrate for thin film inductors comprising the steps of:blending raw materials to meet a composition of di-iron trioxide: 40 to55 mol %, nickel oxide: 5 to 35 mol %, zinc oxide: 10 to 40 mol %, andbismuth trioxide: 150 to 750 ppm; molding the blended material;sintering the molded material; and applying hot isostatic pressing tothe sintered article.
 20. The manufacturing method as claimed in claim19, wherein the blending step includes blending raw materials to meet acomposition of di-iron trioxide: 40 to 55 mol %, nickel oxide: 15 to 30mol %, zinc oxide: 20 to 40 mol %, and bismuth trioxide: 150 to 750 ppm.21. The manufacturing method as claimed in claim 19, wherein said methodfurther comprises a step of annealing the sintered article afterapplying hot isostatic pressing, and a step of surface-lapping theannealed article with the amount of lapping at least 5 μm.
 22. Amanufacturing method of a ferrite substrate for thin film inductorscomprising the steps of: blending raw materials to meet a composition ofdi-iron trioxide: 40 to 55 mol %, nickel oxide: 5 to 35 mol %, zincoxide: 10 to 40 mol %, cupric oxide: 5 to 10 mol %, and manganesedioxide: 0.5 to 2 mol %; molding the blended material; sintering themolded material; and applying hot isostatic pressing to the sinteredarticle.
 23. The manufacturing method as claimed in claim 22, whereinthe blending step includes blending raw materials to meet a compositionof di-iron trioxide: 40 to 55 mol %, nickel oxide: 15 to 30 mol %, zincoxide: 20 to 40 mol %, cupric oxide: 5 to 10 mol %, and manganesedioxide: 0.5 to 2 mol.
 24. The manufacturing method as claimed in claim22, wherein said method further comprises a step of annealing thesintered article after applying hot isostatic pressing, a step ofsurface-lapping the annealed article with the amount of lapping at least5 μm.